1. Field of the Invention
The present invention generally relates to a system for controlling an intake air to be supplied to an internal combustion engine, and more particularly to a system for adjusting an amount of air to be introduced to a secondary intake air passage that connects to a main intake air passage at a position downstream of a throttle valve when the engine is operating in a start-up condition.
2. Description of the Related Art
Some types of engines have a main intake air passage and a secondary intake air passage. Usage of the main and secondary intake air passages is controlled by an air flow control unit. The secondary intake air passage connects to the main intake air passage such that the secondary intake air passage bypasses a throttle valve provided in the main intake air passage. The secondary intake air passage is therefore a bypass passage. During a start-up operation of the engine, the air flow control unit causes the intake air to flow in the secondary intake air passage only, and this intake air is only supplied to the engine. The air flow control unit includes a thermo-sensitive valve located in the secondary intake air passage. The thermo-sensitive valve is heated by a PTC (positive temperature coefficient) heater. The thermo-sensitive valve has wax therein. During a cold start-up of the engine, the wax is in a solid state so that the thermo-sensitive valve is in an open condition. As the PTC heater heats the thermo-sensitive valve (more precisely, the wax) and the temperature of the wax rises, the wax melts and the thermo-sensitive valve gradually (or correspondingly) closes. With this temperature-dependent movement of the thermo-sensitive valve, the intake air flowing in the bypass passage is gradually reduced during the warm-up period of the engine. Accordingly, the engine revolution speed is controlled to an appropriate idling speed during the start-up.
The PTC heater is a ceramic element having a positive temperature-electric resistance characteristic. The PTC heater presents a low resistance at a low temperature, and a high resistance at a high temperature. Referring to FIG. 1 of the accompanying drawings, the air flow control unit will be described briefly. A CPU (central processing unit) 2 and a drive transistor 3 are provided in an ECU (engine control unit) 1, and the PTC heater 4 and the resistance 5 are provided externally of the ECU 1. The PTC heater 4 and resistance 5 creates a DC circuit. When the CPU 2 detects the start of the engine from, for example, the engine revolution speed, the CPU 2 turns the drive transistor 3 on. Upon turning on of the drive transistor 3, a power source voltage VB is applied to the DC circuit of the PTC heater 4 and resistance 5. As a result, a current flows through the PTC heater 4 and resistance 5, and the PTC heater 4 radiates heat. The resistance 5 is provided for adjusting the current flowing through the PTC heater 4. If the resistance value of the resistance 5 is high, the temperature increase of the PTC heater 4 becomes gentle and the opening/closing movement of the thermo-sensitive valve becomes gentle. The resistance value of the resistance 5 is decided by a manufacturer of the air flow control unit such that an opening/closing movement designed by the manufacturer is realized. Thus, the resistance value of the resistance 5 is fixed when the air flow control unit is manufactured.
The ECU 1 is manufactured separately from the thermo-sensitive valve. Therefore, in order to decide the opening/closing movement characteristic of the thermo-sensitive valve, the resistance 5 should be provided outside the ECU 1. Further, if the resistance value of the resistance 5 is fixed, the opening/closing movement characteristic of the thermo-sensitive valve is also fixed. Changing the opening/closing movement characteristic of the thermo-sensitive valve is therefore troublesome.